1. Field of the Invention
The present invention relates to a thermal printer.
2. Description of the Related Art
A thermal printer provided with a heat-sensitive printing section including a thermal head and a platen has a relatively small number of parts and is easily downsized and, thus, is widely adopted as a printer attached to a cash register, a portable terminal unit, an ATM (automated-teller machine) and the like. In a thermal printer of this type, it is known that the platen, acting as a back-up support to permit the thermal head to surely perform stable printing on a printing sheet (or a heat-sensitive paper), is detachably mounted on a frame supporting the thermal head (see, e.g., Japanese Unexamined Patent Publication (Kokai) No. 2002-120389 (JP-A-2002-120389)).
The thermal printer with the detachable platen has an advantage in that certain work, such as the setting of supplied or exchange new printing sheets in a printing stand-by state or the removal of a printing sheet caught or jammed in a printing section during a printing operation, can be performed easily and quickly by removing the platen from the frame. In this connection, as described in JP-A-2002-120389, the thermal printer with the detachable platen may include an openable/closable frame structure (generally referred to as a clamshell structure) that has a first frame member supporting a printing sheet in a rolled form (i.e., a roll of paper) and a second frame member pivotably joined to the first frame member and cooperating with the first frame member to define a rolled-paper housing space. In this arrangement, the thermal head is disposed on the first frame member acting as a stationary base and the platen is disposed on the second frame member acting as an openable/closable cover, so that an openable/closable printing section is thus configured.
In the thermal printer with the detachable platen, in order to ensure a contact pressure between the thermal head and the platen, which is required for continuous printing on the printing sheets, an elastic member is typically arranged between the frame and the thermal head so as to apply an urging or biasing force to the thermal head in a direction making contact with the platen mounted to the frame (i.e., disposed at a proper back-up support position). The frame supports the thermal head, adapted to receive the biasing force of the elastic member, shiftably over a predetermined stroke in opposite directions toward and away from the platen located at the proper back-up support position. As a result, a required contact pressure is generated between the thermal head and the platen, for absorbing dimensional or positional errors thereof and establishing a stable printing performance capable of following a variation in thickness of the printing sheets.
The conventional thermal printer with the detachable platen is configured such that the biasing force of the elastic member for ensuring the required contact pressure between the thermal head and the platen is continuously applied to the thermal head, even in a state where the platen is removed from the frame. In other words, during the shifting motion of the thermal head within the predetermined stroke on the frame, the thermal head always receives the biasing force of the elastic member. In this configuration, when the platen is removed from the frame, the thermal head may be deformed due to the biasing force of the elastic member continuously applied to the thermal head.
The thermal head typically has a construction wherein a heat-generating element is arranged in a linear form on a front surface of a substrate made of a rigid material, such as a ceramic, and the substrate is fixed to a metallic carrier plate having a heat-radiating function with the front surface provided with the heat-generating element being exposed. The carrier plate is provided with axles at longitudinal opposite ends defined in an extending direction of the heat-generating element, and is supported by the frame shiftably over a predetermined stroke with the axles being inserted into guide grooves formed in opposite side plates of the frame. When the platen is removed from the frame, the thermal head having the above-described opposite-ends supported structure is adapted to be continuously subjected to the biasing force of the elastic member, in a state where the axles at the opposite ends of the carrier plate are engaged with the peripheries of the corresponding guide grooves at the limit of a shifting stroke. As a result, a central region of the carrier plate of the thermal head may be deflected (or bent) so as to convexly protrude at the side of the heat-generating element.
The above-described problem of the bending of the thermal head becomes more apparent, as the thermal head is enlarged in a paper-width direction so as to follow the requirement of increasing in width of the printable area in the printing sheet. This is because, even if the paper width is increased, a contact pressure per unit area, required for a stable printing, does not change and, as a result, the biasing force of the elastic member must be increased. If the carrier plate of the thermal head is bent, the substrate made of the rigid material may be peeled off at the opposite ends thereof, defined in the extending direction of the heat-generating element, from the carrier plate so as to rise above the latter. Further, as the period of the removal of the platen increases, the bend of the carrier plate of the thermal head may possibly result in a plastic deformation. The possible plastic deformation of the carrier plate affects a printing quality and, as a result, the thermal head may have to be exchanged with a new one.
The bending of the thermal head also influences the work for mounting the platen to the frame. More specifically, if the platen is handled to be mounted to the frame in a state where the thermal head is bent, the surface (typically made of a rubber) of the platen may possibly be damaged or deformed due to a forcible interference with the thermal head, even when the bend is due to an elastic deformation. The damage or deformation of the surface of the platen also affects a printing quality and, therefore, the platen may have to be exchanged with a new one. In a case where the rigidity of the thermal head is increased by, e.g., increasing the thickness of the carrier plate, to solve the above-described problem relating to the bend of the thermal head, it is, on the other hand, required to consider the outer dimensions, internal space dimensions, assembling workability, etc., of the thermal printer and, therefore, it is often difficult to increase the rigidity of the carrier plate under certain structural restrictions.